A number of complementing schemes for producing isotopically selective photoexcitation in a process for isotope separation such as uranium enrichment are known in the art including the early teaching of the technique in U.S. Pat. No. 3,772,519 in which it is appreciated that selective excitation may be achieved by a finely tuned beam of laser radiation applied to an environment of vapor particles. It has been additionally suggested that the excitation radiation may be chirped or frequency swept over the bandwidth of isotopically selective absorption for the photoexcitation transition as illustrated in Belgian Pat. No. 828,557, and U.S. patent application Ser. No. 729,893, filed Oct. 6, 1976, now U.S. Pat No. 4,088,898 all commonly assigned with the present application.
Recent investigations of the processes of excitation, particularly in the uranium atom and of the desired isotope type, U-235, has led to the development of some specific parameters for the excitation, and for frequency sweeping which permits more effective and economical isotope separation techniques. These are particularly applied in the case where more than a single step of photoexcitation is employed and where excitation from plural low-lying initial energy states in desired as, for example, discussed in commonly assigned Belgian Pat. Nos. 807,118 and 816,057.
In particular, it is known that laser radiation produced by a tuned cavity consists of a number of mode lines or distinct frequencies separated by up to 100 MHz or more within the laser bandwidth. Within the space between modes substantially no useful radiation occurs. At the same time, the absorption frequency for a selected isotope in a mixture of many particles of that and other isotope types includes a bandwith which, while distinct from the absorption frequencies for other isotopes is nevertheless spread over a range of frequencies. This is due to various broadening or degeneracy characteristics of the many individual particles of that one isotope type. Thus a perfect match is impossible between the discrete frequencies of the individual modes of excitation radiation and all the particles of the selected isotope type. A near miss of this sort does not prevent excitation of the particles however. A closeness of this sort is sufficient to permit excitation probabilities effective for isotope separation as specified in the above-identified patents. Closeness is assured by the presence of a number of mode lines distributed over the absorption frequency range for the many particles.